Circuit for conversion from (rms) a.c. to d.c.

ABSTRACT

A multijunction thermal convertor of the differential type is employed to obtain energy balance between a D.C. input voltage and an A.C. input voltage in order to determine the RMS value of the A.C. voltage. Amplified feedback from the output of the convertor is used to maintain this balance, by controlling the gain of an amplifier in the A.C. input. The value of this gain in relation to the D.C. input voltage is a measure of the desired (RMS) A.C. input voltage, and such value is expressed as a D.C. output voltage. The system has greater accuracy than previous systems for achieving a similar measurement.

United States Patent Malcolm [451 Sept. 5, 1972 [54] CIRCUIT FORCONVERSION FROM (RMS) A.C. TO D.C.

[72] Inventor: Ian Malcolm, Smiths Falls, Ontario,

Canada [73] Assignee: Guildline Instruments Limited,

Smiths Falls, Ontario, Canada [22] Filed: Aug. 30, 1971 [21] Appl.No.:176,192

Primary Examiner-William H. Beha, Jr. Attorney-Peter Kirby et al.

[57] ABSTRACT A multijunction thermal convertor of the differential typeis employed to obtain energy balance between a D.C. input voltage and anA.C. input voltage in order 52 US. Cl. ..321/1.5, 324/105, 324/106, todetermine the RMS value of the AC voltage-- 328/144 plified feedbackfrom the output of the convertor is 51 Int. Cl. ..H02m 6011 used tomaintain this balance, by Controlling the gain [58] Field of Search..321/1.5- 324/105 106' of amplifier in The value of this g gain inrelation to the D.C. input voltage is a measure of the desired (RMS)A.C. input voltage, and such [56] References Cited value is expressed asa D.C. output voltage. The system has greater accuracy than previoussystems for UNITED STATES PATENTS achieving a similar measurement.

3,435,319 3/1969 Richman ..32l/l.5 4 Claims, 2 Drawing Figures GA/u (01v7904 7 AMPL/F/E/P 3 V5 AMPUF/E/Q AMlL/F/EE ./A MfiL/F/ze /V3 l9 ac.fisFfilea-wca V0.4 7A E 80 9C5 CIRCUIT FOR CONVERSION FROM (RMS) A,C. TODC.

tion Thermal Convertor published in the Proceedings 1 LEE. Vol. 112, No.4, Apr., 1965, at page794 et seq.

In my US. Pat. application Ser. No. 878,526 filed Nov. 20, 1969, thereis described a multijunction thermal A.C./D.C. convertor consisting of ahelix of resistive wire, the helix having a triangular cross section andthe wire being copper plated on two sides of the helix to form two rowsof thermo-electric junctions. One of the forms of convertor disclosed insaid application is of the differential type in which a pair of heaterassemblies (which also form support elements) each extend along arespective one of two apices of the helix to heat respective rows of thethermoelectric junctions. The third apex of the helix is smoothlycurved, the turns of the helix at this third apex being adhesivelysecured to a complementarily shaped recess in a heat sink. Assumingsymmetry, equal power supplied to each heater will generate zero outputfrom the. helix as a whole, since the voltages generated by the two rowsof thermo-electric junctions balance each other.

The object of the present invention is to provide a circuit using aconvertor, preferably one of this differential multijunction type, thesystem to function as an A.C. to DC. transformer with true RMS response,and with greater accuracy than that obtainable from currently availabledevices.

Further objects of the preferred form of the invention are to providesuch a circuit having a high input impedance, together with fast andconvenient operation.

A manner of carrying the invention into practice is illustrateddiagrammatically in the accompanying drawings, in which:

FIG. 1 shows a perspective view of a fragment of a thermal,multijunction convertor; and

FIG. 2 illustrates a conversion circuit constructed according to thepresent invention.

The convertor 9 shown in FIG. 1 is of the type disclosed in my saidprior patent application. An elongated aluminum mount 10 is formed witha central, part cylindrical recess 11, flat surfaces 12 on each of therecess 11, and inclined sides 13. A helix 14 of relatively highresistance (Constantan) wire has a lower apex 15 that complements theshape of the recess 11 and is secured within this recess by an adhesive.Two upper apices 16 and 17 of the helix are supported by heaterassemblies 18 and 19 respectively. While these heater assemblies maytake different forms, each preferably consists of a bifilar heater wirehaving two portions 20 and 21 twisted together and a bifilar thermalwire having two portions 22 and 23 intertwined with each other and withthe heater wire portions 20 and 21. The heater wire portions willpreferably be of Manganin (Mn-12%; Ni-4%; Cu-84%) and the thermal wireportions of copper, both being coated with a suitable enamel to obtainthe necessary electrical insulation, while maintaining close thermalcontact between the wires. The function of the thermal wire portions isto enhance the temperature uniformity of each heater assembly along itslength. At the far end of each heater assembly, not shown in FIG. 1, theheater wire portions are looped together so that current introduced intothe portion 20 from a lead 24 will flow the length of the assembly andback along the heater wire portion 21 to the lead 25.

The end 26 of the helix 14 is soldered to a lead 27, and a similar leadis connected to the remote end of the helix (not shown). The helix willtypically have about 200 turns of wire, these turns being carefullywound with uniformed spacing from each other. They must not touch eachother, as this wire bears no insulation.

Portions 30 of each turn of the helix are coated (plated) with a lowresistant metal, such as copper, and the portion 31 of each turn isuncoated. The plated portions of the helix are shown stippled in thedrawing, for ease of illustration. It will be apparent that the coppersurface layers of the plated portions 30 effectively short circuit theportions of the high resistance helix beneath them and form with theunplated portions 31 a row of thermo-electric junctions 32, such rowsbeing in intimate thermal contact with the heater assemblies 18 and 19,respectively. The mount 10 acts as a heat sink.

If the two heater assemblies 18 and 19 are heated identically, i.e. bythe same power, the voltages generated in the two rows of junctionsbalance so that there is no voltage output across the two ends of thehelix.

In FIG. 2, the convertor 9 is shown in diagrammatic form, the heater 18being heated by an alternating current input voltage. The output of thehelix of the convertor 9 is fed to an amplifier 35 of very high(theoretically infinite) gain. The DC. powered heater 19 is suppliedwith a voltage V3 from a DC. reference voltage source 36, the voltageapplied to the A.C. heater 18 being adjusted to achieve balance in theconvertor 9. To achieve this effect the output of the amplifier 35 isconnected to the gain control 37 of an input amplifier 38 that receivesthe input A.C. voltage V1, the output of the amplifier 38 passingthrough a further (unity gain) amplifier 39 to the heater 18. Thisarrangement ensures that the amplifier 35 always drives the gain controlof the amplifier 38 in such a way as to cause equivalent heating effectsin the heaters 18 and 19. The ratio between the input voltage V1 and thereference voltage V3 is then equal to the gain G of the amplifier 38. Ifthis gain can be determined, in relation to the reference voltage V3,the RMS value of the input voltage V1 will be known.

The amplifier 38 is a phase compensated amplifier giving equal A.C. andDC. gain. This gain can therefore be found by injecting a DC. voltageinto the input of the amplifier 38 and measuring the resulting D.C.level at its output. This DC. output component must, however, be removedbefore the voltage is applied to the heater 18. In practice, the DC.output voltage component is kept constant, while the DC. input isallowed to vary. This is accomplished by using a feedback loop equalsV3/V4. The D.C. component of the output V5 of the amplifier 38, Le. V3,is now subtracted at a second input of the amplifier 39, thus ensuringthat only the A.C. component of V5 is applied to the heater l8.

As has been noted above,

It will be noted that the accuracy is independent of V3, and that a D.C.output voltage V4 is obtained numerically equivalent to the RMS value ofthe A.C. input voltage V1.

It should be noted that the gain G of the amplifier 39 need notnecessarily be unity. This is merely a convenience to obtain directreading at the output voltage V4. In the general case, V4 G V1 (RMS), inwhich case V4 corresponds to V1 (RMS) in the sense of being proportionalthereto rather than exactly equivalent thereto.

1 claim:

1. A circuit for conversion of an (RMS) A.C. input voltage to acorresponding D.C. output voltage, comprising:

a. a convertor having a D.C. input, an A.C. input, and an outputrepresentative of a difference between a D.C. voltage applied to saidD.C. input and the RMS value of an A.C. voltage applied to said A.C.input,

b. means or connecting said D.C. input to a D.C. reference voltage,

c. amplifier means including variable gain control means,

d. means connecting the output of said amplifier means to said A.C.input of the convertor,

e. means for connecting an input of said amplifier means to the A.C.input voltage to be converted,

f. means connecting said output of the convertor to said gain controlmeans to vary the same to bring said convertor output to zero, and

g. means for determining the ratio between the gain of said gain controlmeans and the value of said D.C. reference voltage including means forexpressing said ratio as a D.C. output voltage to pro-1.

vide a measure of the RMS value of said A.C. input voltage to beconverted.

2. A circuit according to claim 1, wherein said means (g) comprise afeed-back loop for determining said gain as a value independent of saidD.C. reference voltage.

3. A circuit for conversion of an (RMS) A.C. input voltage to acorresponding D.C. output voltage, comprising: 1

a. a convertor having a D.C. input, and A.C. input, and an outputrepresentative of a difference between a D.C. voltage ap lied to saidD.C.jn put and the RMS value 0 an .C. voltage apphe to said A.C. input,

b. means for connecting said D.C. input to a D.C.

reference voltage,

c. a first amplifier including variable gain control means and havingequal A.C. and D.C. gain,

means for connecting an input of said amplifier to the A.C. inputvoltage to be converted,

e. a high gain D.C. amplifier including means for connecting a firstinput thereof to said D.C. reference voltage, means connecting theoutput thereof to an input of said first amplifier, and means connectingthe output of said first amplifier to a second input of said high gainamplifier, said first and second inputs of the high gain amplifier beingof opposite polarity,

f. a further amplifier of predetermined gain, including means connectinga first input thereof to the output of said first amplifier, meansconnecting the output thereof to said A.C. input of the convertor, andmeans for connecting a second input thereof to said D.C. referencevoltage, said first and second inputs of the further amplifier being ofopposite polarity,

. and means connecting said output of the convertor to said gain controlmeans to control the same to bring said convertor output to zero andhence cause the output of said high gain D.C. amplifier to provide aD.C. output voltage proportional to the RMS value of said A.C. inputvoltage to be converted.

4. A circuit according to claim 3, wherein the gain of said furtheramplifier is unity, whereby said D.C. output voltage is numericallyequivalent to the RMS value of said A.C. input voltage to be converted.

1. A circuit for conversion of an (RMS) A.C. input voltage to a corresponding D.C. output voltage, comprising: a. a convertor having a D.C. input, an A.C. input, and an output representative of a difference between a D.C. voltage applied to said D.C. input and the RMS value of an A.C. voltage applied to said A.C. input, b. means or connecting said D.C. input to a D.C. reference voltage, c. amplifier means including variable gain control means, d. means connecting the output of said amplifier means to said A.C. input of the convertor, e. means for connecting an input of said amplifier means to the A.C. input voltage to be converted, f. means connecting said output of the convertor to said gain control means to vary the same to bring said convertor output to zero, and g. means for determining the ratio between the gain of said gain control means and the value of said D.C. reference voltage including means for expressing said ratio as a D.C. output voltage to provide a measure of the RMS value of said A.C. input voltage to be converted.
 2. A circuit according to claim 1, wherein said means (g) comprise a feed-back loop for determining said gain as a value independent of said D.C. reference voltage.
 3. A circuit for conversion of an (RMS) A.C. input voltage to a corresponding D.C. output voltage, comprising: a. a convertor having a D.C. input, and A.C. input, and an output representative of a difference between a D.C. voltage applied to said D.C. input and the RMS value of an A.C. voltage applied to said A.C. input, b. means for connecting said D.C. input to a D.C. reference voltage, c. a first amplifier including variable gain control means and having equal A.C. and D.C. gain, d. means for connecting an input of said amplifier to the A.C. input voltage to be converted, e. a high gain D.C. amplifier including means for connecting a first input thereof to said D.C. reference voltage, means connecting the output thereof to an input of said first amplifier, and means connecting the output of said first amplifier to a second input of said high gain amplifier, said first and second inputs of the high gain amplifier being of opposite polarity, f. a further amplifier of predetermined gain, including means connecting a first input thereof to the output of said first amplifier, means connecting the output thereof to said A.C. input of the convertor, and means for connecting a second input thereof to said D.C. reference voLtage, said first and second inputs of the further amplifier being of opposite polarity, g. and means connecting said output of the convertor to said gain control means to control the same to bring said convertor output to zero and hence cause the output of said high gain D.C. amplifier to provide a D.C. output voltage proportional to the RMS value of said A.C. input voltage to be converted.
 4. A circuit according to claim 3, wherein the gain of said further amplifier is unity, whereby said D.C. output voltage is numerically equivalent to the RMS value of said A.C. input voltage to be converted. 